A 55-year-old man admitted on April 1, 2008, for sudden abdominal pain onset whose laboratory data demonstrated apparent inflammation and whose computed tomography (CT) results showed free air and ascites underwent emergency surgery. An ascetic fluid sample submitted for bacteriological examination yielded Streptococcus gallolyticus subsp. pasteurianus, Escherichia coli, Citrobacter freundii, and Bacteroides thetaiotaomicron. He was treated for 6 days with flomoxef (FMOX; 3 g/day) and recovered, being discharged on hospital day 17. This is, to our knowledge, the first case reported in Japan of bacterial peritonitis due to S. gallolyticus subsp. pasteurianus.